The invention pertains to electrodynamic force generators commonly referred to as electrodynamic shakers or linear motion transducers.
The linear motion generator evolved from early dynamic loudspeakers. These had a single ended magnetic structure, a moveable coil within the single ended gap, and a load, the loudspeaker cone, immediately adjacent to the coil and attached thereto.
This structure type has certain disadvantages, among which are, high leakage flux in the load area and inefficiency of the magnetic circuit because of high flux leakage.
These disadvantages were overcome by the center gap electromagnetic structures first introduced in the middle 1950's. Permanent magnet variations of the design were produced in the 1960's but suffered from inefficient (70-85%) utilization of the maximum energy product of the best available magnet materials because of failure to magnetize the magnets in a relatively complete magnet structure. A resultant and major disadvantage of these previous designs was increased weight and cost for a given output force capability. That these magnet types must be magnetized in a complete magnet structure to obtain the maximum energy product is well documented in loudspeaker design literature.
Likewise, in prior force generators, early coil and table support systems were patterned after the diaphragm supports commonly used in conventional loudspeakers. However for structures larger than the typical speaker, the diaphragm type of support proved to be inadequate.
Single cantilever beam flexures at opposite ends of a coil and table structure were later employed. The beam was rigidly fixed at both ends and as a result, the motion of the structure followed a curved path rather than the straight line path which is desired.
Still later systems used cantilever springs with rubber in shear at the end away from the table/coil structure with additional rubber shear blocks between the table/coil structure and the fixed body of the force generator which were used to support part of the weight of the table structure and the test package.
A loop type of flexure (ref: U.S. Pat. No. 3,194,992) was developed which satisfied both load support and guidance requirements. A disadvantage of this design, however, was the high weight of the spring which detracted from the weight of test package that could be vibrated. Additionally, there was difficulty in maintaining the required damping in the assembly which consisted of many layers of a thin metallic material in a roll form using friction between layers as the only damping medium.